Discussion in 'Amateur Radio Amplifiers' started by KB2FCV, May 16, 2011.
Just who were the heathkit engineers who designed the SB-220,SB-200, SB-230? They must be dead.
Mike Elliot, W8KRR (SK) was the Heathkit designer for the SB-104 and later the Drake TR-7.
I do not think Mike designed the SB-230.
The Heathkit amplifiers (SB-200, SB-220) were first sold in mid-1960s.
Sindre Torp, LA6OP has the Heahtkit SB-240 prototype that Mike owned.
Heathkit SB-240 prototype HF amplifier (SB-220 version to match SB-104 series)
Well. I'll be darn, first time I ever saw that amp. Probably just more of the same with a little different twist. Even the HL-2200 was an SB-221 in disguise.
Originally Posted by KM1H
When going with the .01 caps the already marginal 10-15M neutralization becomes worse. Those 200pf caps did work to some degree.
Youre trying to tell us that just the .01's will make the amp fully neutralized on 10M, thats pure nonsense.
Please don't change around what I am saying. You know I never said the .01's "neutralize" anything, so let's stop "inventing" statements.
I **think** you are saying or implying the 200 pF caps neutralize the amplifier on ten meters.
A capacitor to ground from the grid cannot neutralize an amp. All it can do is series tune the grid path to ground. This can make the grid grounding get much better at one frequency, and that does improve input-to-output isolation at one frequency range, but all other frequency ranges get worse and the lower in frequency we go the worse it gets! It is only near and above the SRF that grid to chassis impedance is reduced.
With 200 pF on ten meters, it would take ~.16 uH of inductance from the grid element through tube leads and the cap to the chassis. That would take about six to seven inches of .05 inch diameter grid lead with 200 pF. The actual series resonance of the 572 and capacitor in the SB200 is up around 70 MHz or so. You'll see a decrease in feed through from maybe 60 MHz to just up above the FM band. Below 40 MHz or so the feed through gets worse, not better. Above the FM band not much changes.
Second, that capacitor was put there because Bill Orr used to call everyone and push them into using his "super cathode drive system". I know, because he used to call me about it. He called Heath about the SB1000 not having those magical caps, and he complained when he found out the Warrior II was not going to have them. he complained about the AL80 series also, and the the AL82. He wrote letters to everyone. He had a fetish about small mica capacitors from grid to ground.
The alleged benefit Orr touted was negative feedback, but because grid - cathode operating impedance was so low using that 200 pF cap just really adds feedback that shifts phase and level with grid current. That is never good, and it actually increases IMD. It isn't necessary to guess about this or even debate it. It can, and has, all been measured. We ran IMD, feed through, and stability tests because of all the pressure. I was there in the middle of it all.
The ONLY way to actually neutralize an amp is to add feedback that cancels feed through, and that requires stable broad bandwidth phase inversion and a stable capacitors. I know the circuit you are thinking of, the one used at times in monoband VHF amps where the grid is series tuned to chassis, but that actually is not neutralization. It is simply making the grid to chassis path series resonant so the "ground" is at the grid itself. That's a single band system, and is not actually neutralization. To get down to ten meters in the 200 would take 800 pF more or less, depending on wiring and the tube brand.
I think what you might have seen is the mica cap does help stabilize the amp by grounding the grid better on lower VHF, so if the wiring is sloppy or the parasitic suppressor not up to snuff the cap can help....but it just kills the grid grounding below mid-HF and even hurts it on ten meters.
Frankly, no one in their right technical mind would add 200 ohms of reactance in series with a grid that has dynamic resistance of a few hundred ohms on positive grid to cathode peaks, and nearly open circuit on negative grid to cathode peaks. Bill just pushed so hard it was easy to submit.
We looked at this stuff extensively around the early 1980's.
Tom, please stop dancing.
From your earlier post it appears that you meant going to .01's was a cure-all.
I didnt bother reading much of your last post as it didnt contain much of interest about 10M neutralization.
Fact: With just the 200pf caps the amp is better neutralized than with the .01's.
Fact: with just the 200's the 10M output is lower, 600W in the last amp I worked on. The off the dip Ip was 600ma.
Fact: with .01's the output was 700W but at a 700ma off the dip Ip.
Fact: With full neutralization and .01's the power was 750W with an at the dip 550ma.
Fact: Ive been using .01's and neutralization since the early 90's and you poo-pooed it on the old AMPS forum.
Fact: Ive worked on 300-400 SB-200's since they came out in 1963; most are 6M conversions.
Well, you know more facts than people who design amps and who have been an active part of the industry do. No point in debating someone who knows more than the people who design the amps knew, especially when they know so much they don't bother reading...
Since most of your comments were a decades long rant about Bill Orr which Ive read enough times to memorize, I just skimmed it fast.
The rest was nothing new and already established in discussions over decades; the SB-200 is too simple to fix unlike a few amps that are currently on the market
And, no, I wasnt referring to the lumped LC series tuned screen grid which was mentioned in print back in the old days. Ive only seen it used once.
That's where the idea in the SB200 came from Carl. I can't help that. It is just how it was.
I must not have made my point well. Let me try again...
A capacitor from grid to ground does not neutralize the amp, although I have seen it called that in some Ham books from the past. All it does is interact with the series inductance of the grid to series tune the grid to ground. It forms a simple tuned circuit, with the grid lead inductance as the inductor.
What this does is make the grid to chassis impedance very low at the frequency range where the grid lead resonantes with the capacitor. With the inductance of the 572B and external leads, this is around 60 MHz or so (with short capacitor leads). So technically it does improve grid grounding quite a bit around six meters....but it is absolutely not neutralization and it makes things terribly worse for the amplifier on 80 and 40 meters.
Things are often a balance of many effects, and we have to look at all the effects. In this case we make things a bit better on ten, a whole lot better on six meters, and a whole lot worse on 80 and 40, and even worse on 20 and 15.
If our focus is only on six meters and we never look at the effects on 80 or 40, we might think that 200 pF cap is the greatest thing since sliced bread. If we look at feedthough and output to input interaction on all bands, we would walk away with a different opinion.
What I am saying is with short leads and direct grounding, the SB200 is much better overall at HF then with those silly 200 pF caps. Those caps never did what Orr claimed they did, and actually made things like IMD worse on most bands.
There was a great deal of focus on this around 1980 when Heath was considering revising all the amps. I was right in the thick of it making measurements, and people at Heath confirmed the measurements I made. Thos caps were one of the things that were going to go.
Seems directly grounding the grids on the SB200 not only provides better operational stablility and better performance on bands like 80m, but doing so also provides a much better method of protecting the rig from damage which would otherwise occur from an HV flashover event.
The 200pf caps make no sense.